Although much research has been done in the field of membrane separation and polymers, there have been only incremental advances in gas separation membranes. Increase in selectivity for one gas over another is generally obtained at the cost of simultaneous decrease in permeability and vice versa. Many polymers that exhibited attractive combinations of permeability and selectivity in single gas measurements have failed to show similar properties when tested in gas mixtures due to phenomena such as plasticization, which can sharply reduce selectivity as the concentration of dissolved gas in the polymer increases, typically with increasing feed pressure.
Microporous materials are in the forefront of research because of their potential application into gas storage, separation, catalysis, energy conversion and generation, and microfluidic application, etc. In most of the cases such microporous materials are inorganic (e.g. silica, alumina and zeolites). Recently, advances have been made in the field of coordinated frameworks such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), porous aromatic frameworks (PAFs), and conjugated-microporous polymers (CMPs). Common characteristic of these materials is their microporous structure as well by their high internal surface areas. An overview over such materials can be found in D. F. Sanders et al., “Energy-efficient polymeric gas separation membranes for a sustainable future: A review”, Polymer 54 (2013) 4729-4761.
Polymers are generally considered organic non-porous materials. Internal microporosity may result from chain mobility and substantial chain packing of covalent bonds. Lately, several works have been directed towards high free volume polymers known as microporous or nanoporous organic polymers, e.g., R. Dawson et al., “Nanoporous organic polymer networks”, Progress in Polymer Science 37 (2012) 530-563.
Conventional polymers have well-packed structures. Glassy polymers are comprised of highly distorted structures, e.g., polymers with intrinsic microporosity (PIMs), as, e.g., described in M. M. Khan et al., “Cross-linking of Polymer of Intrinsic Microporosity (PIM-1) via nitrene reaction and its effect on gas transport property”, European Polymer Journal 49 (2013) 4157-4166.
H. B. Park et al. first disclosed in “Polymers with Cavities Tuned for Fast Selective Transport of Small Molecules and Ions”, Science 318 (2007) 254-258 a method for producing thermally rearranged (TR) polymers featuring 0.4-0.9 nm free volume elements in the polymer matrix. Due to the easy synthesis and fabrication, these highly permeable polymers are promising candidates for large scale industrial separation processes, e.g., for carbon dioxide capture and storage (CCS), industrial nitrogen generation, ammonia production, refinery process and natural gas sweetening, which have the potential of minimizing process complexity and reducing energy consumption. These materials exhibit high CO2 permeability, good CO2/CH4 permselectivity and excellent resistance to CO2-induced plasticization. For example, for the fluorinated TRO-1 polymer (TRO=“thermally rearranged oxazol”) of this disclosure was reported a CO2 permeability close to 2000 Barrer (or 1.5·10−14 m2/s Pa) and a CO2/CH4 selectivity of 40, with no evidence of plasticization up to 15 bar.
Thermally rearranged (TR) polymers are usually formed from precursor polyimides or polyamides with an ortho-functional aromatic group void. During the rearrangement the ortho-functional group of the polyimide is chemically linked through an O, N or S moiety of a second group that is lost while leading to the desired PBX. The properties of aromatic polyimides are influenced by this ortho-functional group void.
The TR polymers having polybenzoxazole (PBO) structures described in the above-referenced H. B. Park et al., Science, 2007, article are formed via molecular thermal rearrangement of precursor poly(hydroxyimide)s (PHI), i.e., aromatic polyimides containing hydroxyl groups (or —SH- or —NH-groups) in ortho-position to the imide ring, as shown in FIG. 1 and FIG. 2. Upon heating at high temperatures typically greater than 350° C. in an inert atmosphere (such as N2 or Ar), the aromatic rings thermally rearrange to PBOs with quantitative loss of carbon dioxide. The membrane performance resulting from the TR process is markedly enhanced, following from the unavoidable insolubility of these PBO structures. Starting from a soluble poly(hydroxyimide) and proceeding with the thermal treatment has rendered the industrial processing such as flat membrane production or hollow fiber spinning possible. The procedure is similar in the case of polybenzothiazoles (PBT) and polybenzimidazoles (PBI), as can be seen from FIG. 1, where the two doubly bound O-atoms of the imide group are lost during the rearrangement together with carbon atoms as CO2, whereas X (═O, S, N) is integrated into the heterocycle.
In S. H. Ran et al., “Thermally Rearranged (TR) Polybenzoxazole: Effects of Diverse Imidization Routes on Physical Properties and Gas Transport Behaviors”, Macromolecules 43 (2010) 7657-7667, it was shown that the distortion of the polyimide chain into a rigid-rod polymer during the structural rearrangement in the solid state leads to the formation of microcavities in the range of 0.3-0.4 nm and 0.7-0.9 nm, where smaller sizes could be beneficial for selective transport of different molecules and the larger for gas diffusion. The microporosity in terms of size and distribution of the TR polymers is prone to be controlled by the fabrication conditions in terms of treatment time and temperature, in contrast to commonly used microporous materials.
The TR polymer's combination of exceptional high selectivity and high permeability within the interconnected free volume elements allows to escape the hitherto limiting trade-offs between selectivity and permeability for example for O2/N2, CO2/N2 or CO2/CH4, as well as for gas/liquid or liquid/liquid separations.
For example, natural gas purification is one of the largest gas separation applications in the world. Nearly 100 trillion scf (standard cubic feet), or ca. 2.83 trillion standard cubic meters of natural gas are produced worldwide each year, and approximately 17% of that requires treatment for CO2. While membranes constitute less than 5% of the market, improving membrane permeability, selectivity, and chemical resistance can greatly increase this market share.
Despite the fact that there have been several attempts to improve the properties by using different structures or by studying the effect of the synthesis route in the properties as, e.g., in S. H. Ran et al., Thermally Rearranged (TR) Polybenzoxazole: “Effects of Diverse Imidization Routes on Physical Properties and Gas Transport Behaviors”, Macromolecules, 43 (2010) 7657-7667, the main disadvantage of this kind of polymers is the high temperature necessary to achieve this rearrangement, namely generally >400° C.
The thermal rearrangement or conversion temperature (TTR) greatly influences the polymer and membrane properties and is one of the crucial factors for designing a cost effective thermal treatment process for TR polymer membranes. It has been widely described that the thermal behaviour of polyhydroxyimide by thermogravimetric analysis (TGA) shows two distinct weight losses. The first and wide one appears in the range of 300-500° C. corresponding to the CO2 evolved in the rearrangement to PBO. The second one is due to the decomposition of the polymer backbone at around 500-600° C. Furthermore, thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) provided evidence for the CO2 evolution by detection of the mass weight of 44.
Refining this, Calle et al., “The relationship between the chemical structure and thermal conversion temperatures of thermally rearranged (TR) polymers”, Polymer 53 (2012) 2783-2791, defined three temperatures with significant changes in the first slope in the TGA curve in order to determine TTR. Three different points (TTR1, TTR2 and TTR3) were defined, namely TTR1 as the initial temperature of the weight loss defining the temperature at which polymer chains started the cyclization process, TTR2 as the temperature at the maximum point of weight loss or maximum amount of CO2 evolution and TTR3 as the final temperature, end of the weight loss, marking the completion of the rearrangement process.
In general, the most effective and fastest way to carry out the complete cyclization is by thermal treatment at temperatures higher than TTR2. The temperature used for the rearrangement is influential, since an exponential increase in the conversion rate was shown with the temperature of treatment, finding the maximum increase at T≥TTR2.
Most research efforts and some earlier fundamental studies accordingly showed that imide-to-benzoxazole conversion requires high temperature treatments to produce PBOs with good properties. However, thermal degradation may overlap with the TR process after, especially at long treatment times, resulting in poor mechanical properties of the TR membranes.
Procedures describing attempts to reduce this rearrangement temperature have been reported, e.g. in R. Guo et al., “Synthesis and characterization of Thermally Rearranged (TR) polymers: influence of ortho-positioned functional groups of polyimide precursors on TR process and gas transport properties”, Journal of Materials Chemistry A, 1 (2013) 262-272. Attempts using more flexible monomers and subsequently more flexible polyimides with lower glass transition temperature (Tg) have been reported which lead to a reduction of the rearrangement temperature required for a total PBO conversion of 100° C. (from 450° C. to 350° C.). On the other hand, the flexibility of these membranes lead to a strongly detrimental effect on the final separation properties of the polymer.
Gang Yang et al, “A New Synthetic Route to Benzoxazole Polymer via Tandem Claisen Rearrangement”, Marcromolecules 2001, 34, pages 6545 to 6547, discloses a synthetic route to a soluble aromatic polybenzoxazole through thermal transformation of a precursor polyamide which has isobutenyl bis(aryl ether) moieties ortho to amide nitrogen in polyamide. A Claisen rearrangement of the moieties initially led to the formation of bis(o-amidephenol) linkages, followed by the intramolecular cyclization, with loss of water, to oxazole rings in the resulting polymer.
PBX structures could be obtained also from polyamide derivates, e.g., polyhydroxyamides (PHA) for the production of PBO. It was observed for the structures derived from PHA that the temperature necessary for the thermal rearrangement to transform to the corresponding PBO is lower than for the corresponding derived from PHI. This is possibly due to the higher flexibility around the aromatic amide linkage of the hydroxyl groups in PHA compared to the tertiary amine in PHI, thus PHAs can be thermally rearranged at temperatures usually 100° C. lower than PHIs. Despite this, there is a certain detrimental effect on the separation properties as compared to those derived from PHI.